The impressionists sought to capture the qualities of natural lighting in their artwork and, thus, used quicker strokes in their works. Most famous of the impressionists was Claude Monet. This is oft times confused with post-impressionism or inversely post-impressionists such as Van Gogh are often incorrectly associated with Impressionism. Impressionism started in the mid-19th Century.
Post-impressionism is when figures and subject matter began to distort - the impressionists were concerned more with realism but wanted to capture their subject as quickly as possible. Post-impressionist are more interested in aesthetic aspects of their work such as color and brush stroke over direct interpretation of the subject matter. This is late 19th Century.
Expressionism would be the end of this evolution, when the images become completely distorted and all together subjective. Some of the more famous artists associated with this movement include Gotlib, Kandinsky, de Kooning, and Stella. Expressionism is roughly associated with the turn of/early the 20th century. Sometimes Munch (The Scream) is associated with this group, but his work was more in line with the post-impressionists. Likewise, Pollock and other abstract expressionists (1940s/50s) are sometimes lumped into the Expressionists (de Kooning does make the transition from Expressionist to abstract Expressionist along with some of his colleagues).
Sometimes abstraction is associated with expressionism, but there are other movements that make broader use of abstraction than did the expressionists (such as later cubists, futurists, surrealists and, predominently, the dada'ists).
